12.1.2 Summary of the First and Second Assessment Reports

The first IPCC Scientific Assessment in 1990 (IPCC, 1990) concluded that the
global mean surface temperature had increased by 0.3 to 0.6°C over the previous
100 years and that the magnitude of this warming was broadly consistent with
the predictions of climate models forced by increasing concentrations of greenhouse
gases. However, it remained to be established that the observed warming (or
part of it) could be attributed to the enhanced greenhouse effect. Some of the
reasons for this were that there was only limited agreement between model predictions
and observations, because climate models were still in the early stages of development;
there was inadequate knowledge of natural variability and other possible anthropogenic
effects on climate and there was a scarcity of suitable observational data,
particularly long, reliable time-series.

By the time of the SAR in 1995, considerable progress had been made in attempts
to identify an anthropogenic effect on climate. The first area of significant
advance was that climate models were beginning to incorporate the possible climatic
effects of human-induced changes in sulphate aerosols and stratospheric ozone.
The second area of progress was in better defining the background variability
of the climate system through multi-century model experiments that assumed no
changes in forcing. These provided important information about the possible
characteristics of the internal component of natural climate variability. The
third area of progress was in the application of pattern-based methods that
attempted to attribute some part of the observed changes in climate to human
activities, although these studies were still in their infancy at that time.

The SAR judged that the observed trend in global climate over the previous
100 years was unlikely to be entirely natural in origin. This led to the following,
now well-known, conclusion: "Our ability to quantify the human influence
on global climate is currently limited because the expected signal is still
emerging from the noise of natural variability, and because there are uncertainties
in key factors. Nevertheless, the balance of evidence suggests that there is
a discernible human influence on global climate". It also noted that the
magnitude of the influence was uncertain.

In Section 12.3, a qualitative assessment is made of
observed and modelled climate change, identifying general areas of agreement
and difference. This is based on the observed climate changes identified with
most confidence in Chapter 2 and the model projections of climate change from
Chapter 9.

Next, in Section 12.4, advances obtained with quantitative
methods for climate change detection and attribution are assessed. These include
results obtained with time-series methods, pattern correlation methods, and
optimal fingerprint methods. The interpretation of optimal fingerprinting as
an estimation problem, finding the scaling factors required to bring the amplitude
of model-simulated changes into agreement with observed changes, is discussed.
Some remaining uncertainties are discussed in Section 12.5
and the key findings are drawn together in Section 12.6.